1. Field of the Invention
This invention relates to a highly efficient coding apparatus applicable to a digital video type recording and/or reproducing apparatus (VTR) configured to compress the amount of data of a digital picture signal and record it on a magnetic tape, using a rotary head.
2. Description of the Prior Art
The assignee of the present application has previously proposed a highly efficient coding apparatus, as disclosed in the specification of Japanese Patent Publication No. 144989/1986, which obtains a dynamic range defined by maximum and minimum values of a plurality of picture elements contained in a two-dimensional block and performs coding which is adapted to the dynamic range. Another highly efficient coding apparatus is described in the specification of Japanese Patent Publication No. 92620/1987, which performs coding adapted to the dynamic range of a three-dimensional block consisting of picture elements of areas included in a plurality of respective frames. Moreover, a variable-length coding method is described in the specification of Japanese Patent Publication No. 128621/1987, in which the number of bits used varies in response to the dynamic range so as to maintain the maximum distortion produced upon digitization at a constant value.
The aforementioned highly efficient coding method (called ADRC) which adapts to a respective dynamic range permits significant compression of the amount of data to be transmitted and is therefore suitable for use in a digital VTR. In particular, the variable-length ADRC method can increase the compression rate. However, since the variable-length ADRC method is subject to variations in the amount of transmitted data with the contents of the picture, buffering is required when using a transmission path having a fixed rate, such as in a digital VTR configured to record a predetermined amount of data in one track.
The assignee of the present application has already proposed a buffering apparatus as disclosed in, for example, the specification of Japanese Patent Publication No. 111781/1989, which obtains the frequency distribution of dynamic ranges, converts it into a cumulative type distribution, subsequently obtains the amount of generated data, supplying coding thresholds to the cumulative type distribution, and determines the thresholds such that the amount of generated information does not exceed the transmission rate.
An explanation of buffering is presented below where the bit lengths of picture element codes of variable-length ADRC are (0 to 4). Let the thresholds for coding be T1 to T4 (where T1&gt;T2&gt;T3&gt;T4). Then the bit length is 4 for a picture block having a dynamic range DR of (maximum value to T1), the bit length is 3 for a picture block having a dynamic range DR of (T1 to T2), the bit length is 2 for a picture block having a dynamic range DR of (T2-1 to T3), the bit length is 1 for a picture block having a dynamic range DR of (T3-1 to T4), and the bit length is 0 (no picture element code is transmitted) for a picture block having a dynamic range DR of (T4-1 to the minimum value). 32 sets of threshold combinations for the thresholds T1 through T4 are originally prepared. These sets of thresholds are so arranged that the use of the first set of thresholds results in the maximum amount of generated information, and the use of the 32nd set of thresholds results in the minimum amount of generated information, while gradually and monotonically decreasing the amount of generated information from the first set of thresholds to the 32nd. Respective sets of thresholds are distinguished by threshold codes of five bits.
A table of the frequency distribution for the occurrence of dynamic ranges DR of a number of picture blocks contained in a two-frame period of the entered video data is made. This processing may be carried out by adding +1 to the data to be written in each address of a memory (RAM), where the address is the dynamic range DR. By accumulating the frequency of each address, the table of the frequency distribution becomes a cumulative type. The amount of generated information can be obtained from the application of the above-indicated sets of thresholds to the cumulative type frequency distribution table. A set of thresholds is selected so that the amount of generated information in the two-frame period does not exceed the capacity of the transmission path. ADRC coding is then performed using the selected set of thresholds.
The assignee of the present application has also proposed a process enabling further compression of the amount of information by combining ADRC of a three-dimensional block and frame-dropping processing (see the specification of Japanese Patent Publication No. 9394/1988). In this process, when the three-dimensional block is a still picture block, an average of the picture elements at corresponding positions in a plurality of areas which form the three-dimensional block is obtained and transmitted, thereby to compress the picture element data of the picture block by a half. An MDT flag indicative of whether frame-dropping processing has been performed is transmitted to the receiver (reproduction) side.
Even in the case of a highly-efficient coding system combining such three-dimensional ADRC and frame-dropping processing, buffering is utilized. As a buffering method of this type, the assignee of the present application has already proposed several methods as disclosed in Japanese Patent Publication Nos. 299587/1989, 299588/1989 and patent application No. 183781/1988 wherein it is taught that the amount of information can be controlled by controlling both the above-mentioned thresholds in the level direction of the dynamic range DR and a threshold determining whether frame-dropping processing should be performed. The threshold for determining whether frame-dropping processing should be performed is called a movement threshold.
An output signal produced by the above-mentioned combination of ADRC and buffering, when recorded, is converted by a frame segmentation circuit into the form of a recording signal whose sync blocks are serial. Further, the reproduced signal is supplied to an ADRC decoder via a frame desegmentation circuit.
In the case of the above-described variable-length ADRC, the bit length of a bit plane which is a coded output of each picture element is determined for each picture block. Bit plane data are stuffed into sync blocks sequentially to form recording data. In the normal reproduction mode where reproduced data are obtained in serial form, the relationship between the reproduced data and the number of the picture block (position of the picture block) for every two frames is determined on the reproduction side. In contrast, in the picture search mode where the tape is driven at a high speed, the head scans some of the tracks simultaneously, and reproduced data are obtained in a discontinuous form for each sync block unit. Therefore, in the picture search mode it is difficult to properly restore the bit planes at the reproduction side and obtain a reproduced picture.
In addition, in the case of the above-indicated variable-length ADRC method including buffering, since the amount of generated data is controlled in predetermined intervals, e.g. in two-frame intervals, the threshold value for controlling the generated data amount is determined for every two frames. Therefore, the threshold code THR may be transmitted once for every two frames. However, when the threshold code becomes erroneous due to an error generated in the recording or reproducing process, the coded data for the two-frame period cannot be decoded.